The present invention relates generally to micro-mechanical devices, and more particularly, to a micrometer sized thermal actuator that is capable of repeatable and rapid movement horizontally across a substrate, vertically off the surface of the substrate, or a combination thereof.
Fabricating complex micro-electro-mechanical systems (MEMS) and micro-optical-electro-mechanical systems (MOEMS) devices represents a significant advance in micro-mechanical device technology. Presently, micrometer-sized analogs of many macro-scale devices have been made, such as, for example, hinges, shutters, lenses, mirrors, switches, polarizing devices, and actuators. These devices can be fabricated, for example, using Multi-user MEMS processing (MUMPs) available from Cronos Integrated Microsystems located at Research Triangle Park, N.C. Applications of MEMS and MOEMS devices include, for example, data storage devices, laser scanners, printer heads, magnetic heads, micro-spectrometers, accelerometers, scanning-probe microscopes, near-field optical microscopes, optical scanners, optical modulators, micro-lenses, optical switches, and micro-robotics.
One method of forming a MEMS or MOEMS device involves patterning the device in appropriate locations on a substrate. As patterned, the device lies flat on top of the substrate. For example, the hinge plates of a hinge structure or a reflector device are both formed generally coplanar with the surface of the substrate using the MUMPs process. One challenge to making use of these devices is moving them out of the plane of the substrate.
Coupling actuators with micro-mechanical devices allows for moving these devices out of the plane of the substrate. Various types of actuators, including electrostatic, piezoelectric, thermal and magnetic have been used for this purpose.
One such actuator is described by Cowan et al. in xe2x80x9cVertical Thermal Actuator for Micro-Opto-Electro-Mechanical Systems,xe2x80x9d v. 3226, SPIE, pp. 137-146 (1997). The actuator 20 of Cowan et al. illustrated in FIG. 1 uses resistive heating to induce thermal expansion. The hot arm 22 is higher than the cantilever arm 24, so that thermal expansion drives the actuator tip 26 toward the surface of the substrate 28. At sufficiently high current, the downward deflection of the actuator tip 26 is stopped by contact with the substrate 28 and the hot arms 22 bow upward. Upon removal of the drive current, the hot arms 22 rapidly xe2x80x9cfreezexe2x80x9d in the bowed shape and shrink, pulling the actuator tip 26 upward, as illustrated in FIG. 2.
The deformation of the hot arm 22 is permanent and the actuator tip 26 remains deflected upward without applied power, forming a backbent actuator 32. Further application of the drive current causes the backbent actuator 32 to rotate in the direction 30 toward the surface of the substrate 28. The backbent actuator 32 of FIG. 2 is typically used for setup or one-time positioning applications. The actuators described in Cowan et al. are limited in that they cannot rotate or lift hinged plates substantially more than forty-five degrees out-of-plane in a single actuation step.
Harsh et al., xe2x80x9cFlip Chip Assembly for Si-Based Rf MEMSxe2x80x9d Technical Digest of the Twelfth IEEE International Conference on Micro Electro Mechanical Systems, IEEE Microwave Theory and Techniques Society 1999, pp. 273-278; Harsh et al., xe2x80x9cThe Realization and Design Considerations of a Flip-Chip Integrated MEMS Tunable Capacitorxe2x80x9d 80 Sensors and Actuators, pp. 108-118 (2000); and Feng et al., xe2x80x9cMEMS-Based Variable Capacitor for Millimeter-Wave Applicationsxe2x80x9d Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C. 2000, pp. 255-258, disclose various vertical actuators based upon a flip-chip design. During the normal release etching step, the base oxide layer is partially dissolved and the remaining MEMS components are released. A ceramic substrate is then bonded to the exposed surface of the MEMS device and the base polysilicon layer is removed by completing the etch of the base oxide layer (i.e., a flip chip process). The resultant device, which is completely free of the polysilicon substrate, is a capacitor in which the top plate of the capacitor is controllably moved in a downward fashion toward an opposing plate on the ceramic substrate. The device is removed from the polysilicon substrate because stray capacitance effects of a polysilicon layer would at a minimum interfere with the operation of the device.
Lift angles substantially greater than forty-five degrees are achievable with a dual-stage actuator system. A dual-stage actuator system typically consists of a vertical actuator and a motor. The vertical actuator lifts the hinged micro-mechanical device off of the substrate to a maximum angle not substantially greater than forty-five degrees. The motor, which has a drive arm connected to a lift arm of the micro-mechanical device, completes the lift. One such dual-stage assembly system is disclosed by Reid et al. in xe2x80x9cAutomated Assembly of Flip-Up Micromirrors,xe2x80x9d Transducers ""97, Int""l Conf. Solid-State Sensors and Actuators, pp. 347-350 (1997). These dual stage actuators are typically used for setup or one-time positioning applications.
The present invention is directed to a micrometer sized, multi-directional thermal actuator capable of repeatable and rapid displacement in a substantially horizontal direction, a substantially vertical direction, or a combination thereof. In some embodiments, the thermal actuator can be displaced radially in substantially any direction relative to an unactivated position, where radial refers to a direction generally perpendicular to the longitudinal axes of the beam.
In one embodiment, the multi-directional thermal actuator constructed on a surface of a substrate includes first, second, and third beams cantilevered from an anchor at first ends to extend generally parallel to the surface of the substrate in an unactivated configuration. The first, second, and third beams are not coplanar. A member mechanically couples distal ends of the first, second, and third beams. A first circuit comprises at least the first beam, whereby application of current to the first circuit displaces the member in a first radial direction. A second circuit comprises at least the second beam, whereby application of current to the second circuit displaces the member in a second radial direction. A third circuit comprises at least the third beam, whereby application of current to the third circuit displaces the member in a third radial direction.
In one embodiment, a grounding tab electrically couples one or more of the beams to the substrate. A resistance can optionally be located between one or more of the beams and ground. In one embodiment, the first and second beams comprise a first circuit, the second and third beams comprise a second circuit, and the third and first beams comprise a third circuit. In another embodiment, the first beam and a grounding tab comprise a fourth circuit, the second beam and a grounding tab comprise a fifth circuit, and the third beam and a grounding tab comprise a sixth circuit. The same or different levels of current can be applied to one or more of the circuits simultaneously. The first, second, and third beams can be arranged in a symmetrical or an asymmetrical cross-sectional configuration.
Another embodiment includes a fourth beam cantilevered from an anchor at a first end to extend generally parallel to the surface of the substrate in an unactivated configuration. The fourth beam is mechanically coupled to the member.
In the four beam embodiment, the first and fourth beams comprise a seventh circuit, whereby application of current to the seventh circuit displaces the member in a seventh radial direction. The second and fourth beams comprise an eighth circuit, whereby application of current to the eighth circuit displaces the member in a eighth radial direction. The third and fourth beams comprise a ninth circuit, whereby application of current to the ninth circuit displaces the member in a ninth radial direction. The first, second, third and fourth beams can arranged in a symmetrical or an asymmetrical cross-sectional configuration.
In some embodiments, the multi-directional thermal actuator includes a cold arm having a first end anchored to the surface of the substrate and a distal end mechanically coupled to the member. The beam can be arranged either symmetrically or asymmetrically with respect to the cold arm.
The present invention is also directed to a multi-directional thermal actuator constructed on a surface of a substrate comprising at least three beams each cantilevered from one or more anchors at a first end to extend generally parallel to the surface of the substrate in an unactivated configuration, wherein at least one of the beams is not coplanar with the other two beams. A member mechanically and electrically couples the distal ends of the beams, whereby application of current to a circuit comprising combinations of any two or more of the beams displaces the member in one of three or more non-parallel radial directions, respectively.
In one embodiment, the multi-directional thermal actuator constructed on a surface of a substrate includes two lower hot arms each cantilevered from one or more anchors at a first end to extend generally parallel to the surface of the substrate and two upper hot arms each cantilevered from one or more anchors at a first end to extend generally parallel to the surface of the substrate. The two upper hot arms are arranged above the two lower hot arms, respectively. A member mechanically and electrically couples the distal ends of the upper and lower hot arms.
The actuator exhibits vertical displacement when current is applied to the two lower hot arms or the two upper hot arms. The actuator exhibits horizontal displacement when current is applied to one of the lower hot arms and the upper hot arm located above the lower hot arm. The actuator exhibits both horizontal and vertical displacement when current is applied to any three of the hot arms.
In one embodiment, a cold arm having a first end anchored to the surface of the substrate and a distal end is located generally parallel with the upper and lower hot arms. The cold arm is preferably located symmetrically with respect to the hot arms. In one embodiment, the cold arm is located centrally within a generally rectangular space bounded by the upper and lower hot arms. A flexure is optionally formed in the cold arm near the first end thereof. The flexure comprises at least one of a recess, depression, cut-out, hole, location of narrowed, thinned or weakened material, alternate material or other structural features or material change that decreases resistance to bending in that location. In another embodiment, the cold arm includes a reinforcing member. The reinforcing member can be integrally formed in the cold arm. A metal layer optionally extends along the cold arm to reduce current density.
In one embodiment, the cold arm is electrically isolated from the hot arms. In another embodiment, the hot arms and the cold arm comprise a circuit through which electric current can pass. The actuator exhibits both horizontal and vertical displacement when current is applied to a circuit comprising the cold arm and any one of the hot arms, any three of the hot arms, or two unbalanced sets of arms. A grounding tab can optionally be provided to electrically couple one or more of the hot arms to the substrate.
A plurality of multi-directional thermal actuators can be formed on a single substrate. At least one optical device can be mechanically coupled to the multi-directional thermal actuator. The optical device comprises one of a reflector, a lens, a polarizer, a wave guide, a shutter, or an occluding structure. The present invention is also directed to an optical communication system including at least one optical device.